A roller cone rock bit is the cutting tool used in oil, gas, and mining fields to break through the earth formation to shape a well bore. Load and motion of the bit are transferred to the bearings inside three head and cone assemblies. For the bit where a journal bearing is employed, the main journal bearing is charged with as much as 80 percent of the total radial load. The main journal bearing is composed of the head (as the shaft), the bushing, and the cone (as the housing). Due to high load (>10,000 lb) and low speed (70-250 rpm), the bearing operates in the boundary or the mixed lubrication regime in which the head is not completely separated from the bushing. Load is supported by both asperity contact and hydrodynamic pressure. The grease constrained among those asperities is the effective lubricating medium and its amount is quite limited. It is not unusual for the bearing to experience grease starvation in the contact zone. Thereafter the bearing often undergoes scoring, scuffing, even catastrophic failure like galling or seizure. It is desirable that more grease be trapped between the head and bushing at the contact interface in order to reduce friction.
Reference is made to FIG. 1A which illustrates a partially broken away view of a typical roller cone rock bit. FIG. 1A more specifically illustrates one head and cone assembly. The general configuration and operation of such a bit is well known to those skilled in the art.
The head 1 of the bit includes the bearing shaft 2. A cutting cone 3 is rotatably positioned on the bearing shaft 2 which functions as a journal. A body portion of the bit includes an upper threaded portion forming a tool joint connection 4 which facilitates connection of the bit to a drill string (not shown). A lubrication system 6 is included to provide lubrication to, and retain lubricant in, the journal bearing between the cone 3 and the bearing shaft 2. This system 6 has a configuration and operation well known to those skilled in the art.
A number of bearing systems are provided in connection with the journal bearing supporting rotation of the cone 3 about the bearing shaft 2. These bearing systems include a first cylindrical friction bearing 10 (also referred to as the main journal bearing herein), ball bearings 12, second cylindrical friction bearing 14, first radial friction (thrust) bearing 16 and second radial friction (thrust) bearing 18.
With reference to FIG. 1B, there is shown an illustration of a partially broken away view of another typical roller cone rock bit. Reference 141 refers to the axis of rotation for the cone 150 on the head shaft 130. Reference 106 refers to central rotating axis of the bit itself. Like reference numbers refer to like or similar parts. It will be noted here that a tapered thrust bearing 16′ is used.
FIG. 2 illustrates a partially broken away view of FIG. 1A showing the bearing system in greater detail. The first cylindrical friction bearing 10 is defined by an outer cylindrical surface 20 on the bearing shaft 2 and an inner cylindrical surface 22 of a bushing 24 which has been press fit into the cone 3. This bushing 24 is a ring-shaped structure typically made of beryllium copper, although the use of other materials is known in the art. The ball bearings 12 ride in an annular raceway 26 defined at the interface between the bearing shaft 2 and cone 3. The second cylindrical friction bearing 14 is defined by an outer cylindrical surface 30 on the bearing shaft 2 and an inner cylindrical surface 32 on the cone 3. The outer cylindrical surface 30 is inwardly radially offset from the outer cylindrical surface 20. The first radial friction bearing 16 is defined between the first and second cylindrical friction bearings 10 and 12 by a first radial surface 40 on the bearing shaft 2 and a second radial surface 42 on the cone 3. The second radial friction bearing 18 is adjacent the second cylindrical friction bearing 12 at the axis of rotation for the cone and is defined by a third radial surface 50 on the bearing shaft 2 and a fourth radial surface 52 on the cone 3.
An o-ring seal 60 is positioned between cutter cone 3 and the bearing shaft 2. A cylindrical surface seal boss 62 is provided on the bearing shaft. In the illustrated configuration, this surface of the seal boss 62 is outwardly radially offset (by the thickness of the bushing 24) from the outer cylindrical surface 20 of the first friction bearing 10. It will be understood that the seal boss could exhibit no offset with respect to the main journal bearing surface if desired. An annular gland 64 is formed in the cone 3. The gland 64 and seal boss 62 align with each other when the cutting cone 3 is rotatably positioned on the bearing shaft. The o-ring seal 60 is compressed between the surface(s) of the gland 64 and the seal boss 62, and functions to retain lubricant in the bearing area around the bearing systems and prevents any materials (drilling mud and debris) in the well bore from entering into the bearing area.
With reference once again to FIG. 1B, the tapered thrust friction bearing 16′ is defined between the first and second cylindrical friction bearings 10 and 12 by a first conical surface 40′ on the bearing shaft 130 and a second conical surface 42′ on the cone 150. The features of FIG. 1B are otherwise generally the same as in FIGS. 1A and 2.
While the surfaces have in some instances been referred to as cylindrical or radial, and have been shown as linear, it will be understood that other surface geometries (for example, non-linear geometries neither parallel with nor perpendicular to the axis 141 of cone rotation, such as toroidal or otherwise curved) as are known to those skilled in the art may used for the bearing and thrust surfaces.
Load in the bearing system is supported by both asperity contact and hydrodynamic pressure. Lubricant is provided in the first cylindrical friction bearing 10, second cylindrical friction bearing 14, first radial friction bearing 16 (or tapered thrust bearing 16′) and second radial friction bearing 18 between the implicated cylindrical and radial surfaces using the system 6. However, it is not unusual for the bearing to experience grease starvation in these surface contact zones of the bearing system. This can result in scoring, scuffing, and even catastrophic failure like galling or seizure. There is a need to retain lubricant in position trapped between the implicated and opposed cylindrical and radial surfaces of the bearing system.
Reference is made to the following prior art documents: U.S. Pat. Nos. 3,839,774 (Oct. 8, 1974), 4,248,485 (Feb. 3, 1981) and 5,485,890 (Jan. 23, 1996): U.S. Publication 2005/0252691 (Nov. 17, 2005); and PCT Publication WO 2007/146276 (Dec. 21, 2007), the disclosures of which are hereby incorporated by reference.